In this work, an instrument for measuring pulsed streaming potentials was constructed and optimized for analytical and teaching applications. This thesis is divided in three chapters, the first one deals with the construction of the instrument, the second describes a microfluidic experiment designed for undergraduate and high school students using this instrument, and the third one shows an application of pulsed streaming potential measurements in the detection of heparin. Streaming potential is the electric field generated when a liquid is forced to flow by a pressure gradient through a channel or other stationary charged surfaces.1,2 These measurements were done in microfluidic channels built with commodity plastics such polycarbonate (PC) and cyclic olefin copolymer (COC). Microfluidics studies the changing behaviors of fluids within small volumes, (nL, pL, fL), or small sizes, (channel size is about 100 nanometers to several hundred micrometers).3 With low level of complexity in instrumentation, low cost, and easy way to implement, the system is ideally suited as a teaching instrument in high 2 school and undergraduate labs. By creating simple experiments with suitable processing time, our goal is to introduce to students several fundamental concepts related to ionic solutions, electrochemical potentials, and charged surfaces. By doing the experiments, students can improve their analytical skills, and problem solving skills. They can learn many useful techniques, such as measuring pH, measuring conductivity, and calculating zeta potential. For these experiments, Polycarbonate (PC) is chosen as microfluidic platform because it is commercially available and the cost is low enough for a school budget. PC microfluidic channels are modified by different charged species, which are the anionic poly (sodium 4-styrenesulfonate) (PSS), the cationic poly(allylamine) hydrochloride (PAH), and bovine serum albumin (BSA). Since the relative polarity of streaming potential is determined by the surface charge, the signal detected is the reverse of streaming potential with different charged modified surfaces. With the same strategy, heparin is detected by real time monitoring adsorption on COC and PC microfluidic channels modified by protamine. The results on the two kinds of channels are compared. For COC channels, linear correlation of initial adsorption rates is found in the range between 0.00074 units/ml and 0.050 units/ml. For PC channels it is between 0.00074 units/mL and 0.074 units/mL. Streaming potential measurements have been useful for determining the charge of such surfaces as capillaries, 1 membranes, 4 and other porous materials.5 There has been no work done using pulsed streaming potential measurements for sensing purposes in microfluidic channels. With our sensing device, no referent electrode is needed since the signal acquisition is made using pulsed flow, so drifting of the measure voltages can be avoided. In addition, no such fluorescent, electrochemical, or radioactive labeling is required for detection.
展开▼
